The present invention relates generally to battery charging systems, and, more particularly, to a battery charger operative to apply a charging current to a battery of a magnitude determined by the temperature level of the battery.
Oftentimes, an electrical device is constructed to be of a design to permit powering thereof by one or more batteries. In some instances, the use of a battery to power the electrical device is necessary when the electrical device is not, or cannot be, positioned proximate to a permanent power supply. In other instances, a battery is used to power the electrical device to increase the portability of the device, as no power cable is required to interconnect the electrical device to a permanent, power supply. Typically, the one or more batteries used to power the electrical device are carried directly with the electrical device.
However, because a battery stores only a finite amount of energy, operation of the electrical device is limited by the energy storage capacity of the battery. Powering of the electrical device by the battery discharges the battery, and, once the battery becomes discharged, replacement of the battery is required to permit continued operation of the electrical device. Increasing the size (and weight) of the battery, while increasing the energy storage capacity of the battery, reduces the portability of the electrical device when the battery is carried therewith. Accordingly, a compromise is made between increased battery energy storage capacity and reduced portability of the electrical device.
A portable or transportable radio telephone is one such electrical device which is typically powered by a battery power supply. The battery power supply is typically carried directly with the radio telephone, and is of size and weight which does not unduly constrain the portability of the radio telephone.
When the radio telephone is operative to send or to receive a modulated signal, the radio telephone typically dissipates power at a rate of up to three watts. Conventional battery power supplies constructed to power such radio telephones are of energy storage capacities permitting power dissipation at such a rate (i.e., operation of the radio telephone) for approximately a one hour time period. If the battery power supply operative to power the radio telephone is not initially fully charged, the operational period during which the radio telephone may be operated at such a power level is, of course, commensurately reduced.
Once the battery power supply has been discharged beneath a certain level, the battery powering the radio telephone must be removed and replaced with a new battery power supply to permit continued operation of the radio telephone.
Rechargeable batteries have been developed and are commercially available. Some of such commercially-available, rechargeable batteries are of constructions designed for use to power radio telephones. The use of rechargeable batteries is advantageous as, after discharge of the batteries, the rechargeable batteries may be recharged, and then reused. Some constructions of rechargeable batteries may be recharged, and reused, up to, and even in excess of, five hundred times.
A rechargeable battery construction used to power a portable radio telephone is typically comprised of several discrete battery cells, connected in a series (or other) connection, and housed within a common housing. The housing, together with the battery cells, is sometimes referred to as a battery pack. For purposes of simplicity, such constructions are most times referred to simply by the term "battery". The instant disclosure utilizes such simplified terminology except when, as shall be noticed hereinbelow, the more precise terminology is required.
Rechargeable battery constructions are comprised of various different materials of construction. For instance, a rechargeable battery may be comprised of a lithium (Li), material, a nickel-cadmium (Ni-Cd) material, or a nickel metal hydride (NiMHO.sub.2) material. Batteries constructed of these different materials of construction exhibit different characteristics during recharging thereof.
Battery charging apparatus is also commercially available to permit recharging of rechargeable batteries. A battery charger comprising such battery charging apparatus is typically comprised of support structure for supporting one or more batteries, and a current source for supplying a charging current which charges the rechargeable battery when suitably positioned upon the support structure. The energy of the charging current applied to the rechargeable battery is converted into chemical energy which is stored by the battery. Application of the charging current to the rechargeable battery over an elapsed period of time permits the rechargeable battery to become fully recharged. The time period required to charge fully the rechargeable battery is dependent upon the battery type, the extent to which the battery has been discharged, and the magnitude of the charging current applied to the battery. Certain battery chargers are of constructions which supply a relatively small magnitude current (such as, for example, a C/10 value wherein C is the one-hour capacity of a battery) to the battery, and battery chargers of other constructions supply a significantly larger current (such as, for example, a C value) to the battery. Application of the large magnitude current to the battery is referred to as fast charging of the battery. Application of the small magnitude current to the battery is referred to as trickle charging of the battery. Several battery chargers are of constructions which permit the application of either the large current (i.e., the fast charging current) or the relatively small current to the battery to recharge the battery thereby.
As the time required to recharge a rechargeable battery is reduced when a large-magnitude charging current is applied to the battery to recharge the battery thereby, charging of the rechargeable battery by the application of a fast charging current thereto is advantageous for reasons of convenience. However, application of the fast charging current to the rechargeable battery causes the temperature of the battery to increase.
A nickel metal hydride battery, in particular, exhibits a rapid temperature increase when a fast charging current is applied to the battery to charge the battery thereby. Many conventional constructions of battery chargers terminate the application of a fast charging current to a rechargeable battery when the temperature level of the battery exceeds a predetermined, threshold value. When applying a fast charging current to a nickel-cadmium battery, the battery typically exhibits a rapid temperature increase only when the battery is, or is almost, fully charged. As a nickel metal hydride battery exhibits a rapid temperature increase prior to complete charging of the battery, application of a fast charging current to a nickel metal hydride battery in such a conventional battery charger would result in termination of the application of the fast charging current to the battery soon after application of the fast charging current commences, and prior to adequate charging of the battery.
Also, a nickel-cadmium battery typically exhibits a voltage drop across output terminals thereof upon complete charging thereof. Accordingly, other conventional battery chargers are of constructions which apply a fast charging current to a battery positioned to be charged thereat until continued application of the fast charging current to the battery results in a voltage decrease across output terminals of the battery. However, when the battery positioned to be recharged is comprised of a nickel metal hydride material, continued application of the fast charging current until the battery exhibits a voltage drop thereacross, would result in excessive temperature levels of the battery undergoing recharging.
Also, when a battery is permitted to be elevated repeatedly to excessive temperature levels, the battery may become damaged, or the battery life of the battery may be reduced.
What is needed, therefore, is a battery charger of a construction operative to apply a fast charging current to a nickel metal hydride battery to charge fully the battery, but which prevents excessive temperature increase of the nickel metal hydride battery during application of the fast charging current thereto.
As a user of a radio telephone may use a rechargeable battery of a nickel-cadmium construction in some instances, and a rechargeable battery of a nickel metal hydride construction in other instances, a battery charger of a construction permitting fast charging of a rechargeable battery of either type of construction would be advantageous.
What is additionally needed, therefore, is a battery charger of a construction operative to apply a fast charging current to a rechargeable battery comprised of either a nickel-cadmium material or a nickel metal hydride material.